As long as you make sure that the battery stays in place, you can just try to boot the FP2 with the case taken off. Indeed, the buttons cannot be interchanged in the old “regular case”, but the issue you might have read about (blocked power button) only affected a batch of the newer “slim cases”. So most likely your reboots are not about the power button, but trying as described cannot hurt.
If I were you and there was a Fairphone Angel nearby, that’s whom I would contact. Have a look at the map or the list of them here:
A Fairphone Angel can test (together with you) if any of your FP2’s modules do not work, e.g. the bottom module which includes the USB (charging + data) port.